Differentiate cached container names depending on application

As the container cache is stored in `sf_app_cache_dir` the class name `ProjectContainer` / `DebugProjectContainer` is causing issues when eg. creating 2 contexts:
```php
require_once(dirname(__FILE__).'/config/ProjectConfiguration.class.php');

$config1 = ProjectConfiguration::getApplicationConfiguration('frontend', 'dev', true);
$context1 = sfContext::createInstance($config1);
$config2 = ProjectConfiguration::getApplicationConfiguration('backend', 'dev', true);
$context2 = sfContext::createInstance($config2);
```
`Fatal error: Cannot redeclare class DebugProjectContainer in .../cache/backend/dev/DebugProjectContainer.php`

The solution is to include app name (instead of "Project") in the container cache class name. So now the class name would be `frontendContainer` / `frontendDebugContainer` etc.